Fermentation and Anaerobic Metabolism

Core Concept

Fermentation is an anaerobic pathway that keeps glycolysis running when oxygen is unavailable. Glycolysis produces a small amount of ATP, but it also converts NAD⁺ to NADH; if NAD⁺ is not regenerated, glycolysis stops. Fermentation solves this problem by oxidizing NADH back to NAD⁺, allowing glycolysis (and its 2 ATP per glucose) to continue. Fermentation does not use an electron transport chain and yields no additional ATP beyond glycolysis. It comes in two main forms — lactic acid fermentation and alcoholic fermentation — and is distinct from anaerobic respiration, which does use an electron transport chain with a non-oxygen final acceptor.

Key Concepts

Why NAD⁺ Must Be Recycled

Glycolysis needs NAD⁺ as an electron acceptor. When oxygen is present, NADH is re-oxidized to NAD⁺ by the electron transport chain. Without oxygen, that route shuts down, and NAD⁺ would run out — stalling glycolysis and cutting off the cell's only anaerobic ATP source. Fermentation's sole purpose is to regenerate NAD⁺.

Lactic Acid Fermentation

Pyruvate is reduced to lactate (lactic acid) using electrons from NADH, regenerating NAD⁺. This occurs in:

• Animal muscle during intense exercise when oxygen delivery lags (causing the temporary buildup of lactate).
• Lactic acid bacteria (e.g., Lactobacillus), used to make yogurt, cheese, sauerkraut, and pickles.

Net yield: 2 ATP per glucose (from glycolysis only).

Alcoholic Fermentation

Pyruvate is first decarboxylated to acetaldehyde (releasing CO₂), which is then reduced to ethanol (ethyl alcohol) by NADH, regenerating NAD⁺. This occurs in yeast and some bacteria, and is the basis of bread-making (the CO₂ makes dough rise), beer, wine, and biofuels.

Net yield: 2 ATP per glucose (from glycolysis only).

Fermentation vs. Anaerobic Respiration

• Fermentation: no electron transport chain; NAD⁺ is regenerated by reducing pyruvate; ATP comes only from substrate-level phosphorylation (glycolysis) — 2 ATP.
• Anaerobic respiration: uses an electron transport chain and chemiosmosis but with a final electron acceptor other than oxygen (e.g., nitrate, sulfate, or ferric iron in certain bacteria/archaea). It yields far more ATP than fermentation, though less than aerobic respiration.

These are frequently confused, but they are fundamentally different in whether an ETC is involved and in how much ATP is made.

Facultative vs. Obligate Anaerobes

• Facultative anaerobes (e.g., yeast, E. coli) can use oxygen when present but switch to fermentation or anaerobic respiration when it is not.
• Obligate anaerobes cannot survive in oxygen and rely on fermentation or anaerobic respiration exclusively.

How It Works

(1) In the absence of O₂, NADH accumulates and NAD⁺ becomes scarce. (2) Glycolysis converts glucose to 2 pyruvate, making 2 ATP and 2 NADH. (3) In lactic acid fermentation, lactate dehydrogenase transfers electrons from NADH to pyruvate, producing lactate and freeing NAD⁺. (4) In alcoholic fermentation, pyruvate decarboxylase removes CO₂ from pyruvate to form acetaldehyde, then alcohol dehydrogenase reduces acetaldehyde to ethanol, again regenerating NAD⁺. (5) With NAD⁺ restored, glycolysis can cycle again, giving the cell a continuous (if small) ATP supply without oxygen. No proton gradient, no ETC, and no additional ATP are involved.

How it works

(1) In the absence of O₂, NADH accumulates and NAD⁺ becomes scarce. (2) Glycolysis converts glucose to 2 pyruvate, making 2 ATP and 2 NADH. (3) In lactic acid fermentation, lactate dehydrogenase transfers electrons from NADH to pyruvate, producing lactate and freeing NAD⁺. (4) In alcoholic fermentation, pyruvate decarboxylase removes CO₂ from pyruvate to form acetaldehyde, then alcohol dehydrogenase reduces acetaldehyde to ethanol, again regenerating NAD⁺. (5) With NAD⁺ restored, glycolysis can cycle again, giving the cell a continuous (if small) ATP supply without oxygen. No proton gradient, no ETC, and no additional ATP are involved.

Common confusions

• "Fermentation and anaerobic respiration are the same." Wrong — fermentation has no ETC and makes only 2 ATP; anaerobic respiration uses an ETC with a non-oxygen acceptor and makes more ATP.
• "Fermentation produces extra ATP." Wrong — all of fermentation's ATP comes from glycolysis (2 per glucose); fermentation only recycles NAD⁺.
• "Lactic acid fermentation only happens in bacteria." Wrong — it also happens in animal muscle during intense exercise.
• "The CO₂ in alcoholic fermentation comes from glycolysis." Wrong — it comes from the decarboxylation of pyruvate to acetaldehyde.
• "Fermentation requires oxygen." Wrong — by definition it occurs without oxygen (anaerobic).

Quick review

• Purpose: regenerate NAD⁺ to keep glycolysis going without O₂.
• Net ATP = 2 per glucose (glycolysis only).
• Lactic acid: pyruvate → lactate; occurs in muscle and lactic acid bacteria.
• Alcoholic: pyruvate → acetaldehyde (+CO₂) → ethanol; occurs in yeast.
• No ETC in fermentation.
• Anaerobic respiration ≠ fermentation (uses ETC, non-O₂ acceptor, more ATP).
• Facultative vs. obligate anaerobes.